As the integration of IC's and LSI's is increasing, the required pin-count is increasing, necessitating fine patterning of leads. Etching is suited to the production of lead frames having leads of fine patterns from lead frame materials. For example, Japanese Patent Application Unexamined Publication No. 3-148856 (1991) discloses production of lead frames by etching a three-layered lead frame material comprising two metal layers of different thicknesses and an aluminum etching stop layer sandwiched therebetween.
In the method, the etching stop layer enables selective etching of one of the two metal layers of different thicknesses. The thicker metal layer is to be the base material of the lead frame and is thick enough to afford the required mechanical strength. The thinner metal layer is to be the bonding areas for connection with IC's and is thin enough to form finely patterned bonding areas. Thus lead frames having both fine bonding areas for connection with IC's and sufficient mechanical strength can be produced by the selective etching.
The etching stop layer of the three-layered lead frame material used in the method, however, is an aluminum layer formed by vapor deposition. The vapor deposition for forming the aluminum layer disadvantageously requires a complicated process, which increases the production cost of the lead frame material.
Further, in the course of producing lead frame by the above-described method, the three-layered lead frame material is exposed to high temperatures when, for example, polyimide films are bonded as an insulating protective films to the surfaces of the lead areas with polyamic acid adhesives by curing the adhesives at 350.degree. C. or higher. The etching stop layer therefore requires a good heat-resistance enough to withstand such a high temperature. For example, nickel layers, which are readily formed by electroplating, cannot act as etching stop layers, because copper diffuses from metal layers into the nickel layers at high temperatures of 350.degree. C. or higher.
In Japanese Patent Application Unexamined Publication No. 5-121617 (1993) is disclosed the production of lead frames from a two-layered lead frame material comprising a first copper layer of 80 to 150 .mu.m thickness and a second aluminum layer of 10 to 50 .mu.m thickness, by selectively plating a third copper layer on the second aluminum layer, and then patterning the first copper layer to form inner leads. Having the aluminum layer as an etching stop layer, the two-layered lead frame material used in the method also involves the same problem as that with the three-layered lead frame material.